Lubricant formulation for high temperature metal forming processes

ABSTRACT

Improved lubricant system useful in metal forming processes. The lubricant formulations contain boron nitride lubricant with graphite additions. These formulations enhance lubricity while maintaining good adherence at elevated temperatures.

TECHNICAL FIELD

The present invention relates generally to lubricant formulations usefulfor elevated temperature metal forming processes such as quick plasticforming, superplastic forming, and warm forming processes.

BACKGROUND OF THE INVENTION

Development of lubricants for elevated temperature metal formingprocesses such as quick plastic forming (QPF), superplastic forming(SPF), or warm forming requires the following features: low coefficientof friction, good adhesion to the blank, uniform application pattern,low cost and ease of removal after forming. Petrochemical basedlubricants are ineffective due to the high temperatures utilized inthese processes. The most commonly used lubricant for processesinvolving such elevated temperatures is boron nitride (BN). However,other lubricants such as graphite, molybdenum disulfide, and magnesiumhydroxide (commonly referred to as milk of magnesia when in aqueoussuspension) have been used.

SUMMARY OF THE INVENTION

This invention is believed to provide advantages and alternatives overprior practices by providing improved lubricant formulations useful inelevated high temperature metal forming processes. The formulationscontain boron nitride lubricant with graphite additions. Theseformulations enhance lubricity while maintaining good adherence to thesurface of the blank.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and which constitutea portion of this specification illustrate an exemplary embodiment ofthe invention which, together with the general description above and thedetailed description set forth below will serve to explain theprinciples of the invention wherein;

FIG. 1 illustrates a cross sectional view of a metal blank and femaleand male shaping tools prior to metal forming;

FIG. 2 illustrates a cross sectional view of a metal blank and femaleand male shaping tools during metal forming;

FIG. 3 illustrates a cross sectional view of a metal blank and femaleand male shaping tools upon completion of metal forming;

FIG. 4 illustrates a cross sectional view of a metal blank and femaleand male shaping tools after removal of the male tool;

FIG. 5 illustrates a cross section view of a metal blank and female andmale shaping tools after removal of the formed metal blank; and

FIG. 6 illustrates the effect of graphite to boron nitride ratio oncoefficient of friction.

While embodiments of the invention have been illustrated and generallydescribed above and will hereinafter be described in connection withcertain potentially preferred procedures and practices, it is to beunderstood and appreciated that in no event is the invention to belimited to such embodiments and procedures as may be illustrated anddescribed herein. On the contrary, it is intended that the presentinvention shall extend to all alternatives and modifications as mayembrace the broad principles of the invention within the true spirit andscope thereof.

DETAILED DESCRIPTION

For ease of reference and understanding, the following description isset forth with respect to a simplified exemplary metal formationprocess. Importantly, it is to be understood that the lubrication systemof the present invention is in no way limited to such a formationprocess. Rather, it is contemplated and intended that the lubricationsystem will be broadly applicable to any number of elevated temperaturemetal formation practices.

Reference will now be made to the various drawings wherein to the extentpossible, like elements are designated by corresponding referencenumerals in the various views. FIGS. 1-5 illustrate schematically asimplified exemplary so called “warm forming” practice utilizing maleand female formation tool members. In the illustrated practice a femalemember 12 includes a mold cavity 32, bottom surface 14, and two opposingwall surfaces 16. A metal blank 22 is placed on top of the female membersuch that a portion of the bottom surface 30 of the metal blank 22 is indirect contact with the outer surface 24 of the female member 12.

FIG. 1 shows the metal blank 22 in position between female member 12 anda complementary male tool 18 such that the upper surface 28 of the metalblank 22 faces the male tool 18. Means (not shown) can be provided toheat both the male tool 18 and the metal blank 22 to a suitabletemperature. The male tool 18 is pressed against the upper surface 28 ofthe metal blank 22. This operation results in creation of an interfaceregion 40 (FIG. 2) between the male tool 18 and the upper surface 28 ofthe metal blank 22 and causes deformation of the metal blank 22 toconform to the shape of the mold cavity 32 of the female member 12.

As shown in FIG. 2, the pressure of the male tool 18 acting on the uppersurface 28 of the metal blank 22 deforms the metal blank downwardlytoward the bottom surface 14 of the female member 12. The bottom surface30 of the metal blank 22 engages a portion of the opposing wall surfaces16 creating an interface region 38 between the female member 12 and thebottom surface 30 of the metal blank 22. In FIG. 3, the male tool 18 haspressed the bottom surface 30 of the metal blank completely against theopposing walls 16 and the bottom 14 of the female member 12. Thisfurther deformation creates a continuous interface region 38 between thefemale member 12 and the metal blank 22 and a continuous interfaceregion 40 between the male tool 18 and the metal blank. At thecompletion of the forming operation, the male tool 18 is lifted fromengagement with the upper surface 28 of the metal blank 22 (FIG. 4) andthe metal blank 22 is removed from the female member 12 (FIG. 5).

As will be appreciated by those of skill in the art, the formationprocess illustrated and described in FIGS. 1-5 is exemplary only. Anynumber of other elevated temperature formation processes may also beutilized. By way of example only, and not limitation, various elevatedtemperature forming processes are disclosed in U.S. Pat. No. 5,819,572to Krajewski, the contents of which are incorporated herein by referencein their entirety.

Regardless of the formation practice utilized, some type of lubricationis typically used during metal forming processes. Lubrication is neededto avoid sticking and thereby facilitate deformation of the work piece.Lubrication also serves to assist in the release of the formed part fromdie and tool members. The lubricant may be applied to the surface of thework piece undergoing deformation and/or to surfaces of the formationtools.

The instant invention provides an improved lubrication system utilizingboron nitride (BN) lubricant with graphite additions to enhancelubricity while maintaining good adherence to the metal blank and/ortool members to which it is applied. In one potentially preferredembodiment, addition of at least 5% and more preferably about 5% toabout 50% graphite to boron nitride results in improved lubricity (lowercoefficient of friction) compared to BN alone. Unlike lubricationformulations which are predominantly graphite, the inventive boronnitride-graphite formulations maintain good adhesion to the metal blankand formation tool surfaces. Other constituents may be present in thelubricant provided they do not materially degrade the features oflubricity and adhesion.

According to the contemplated practice, the inventive boronnitride-graphite formulations can be sprayed on the metal blank or otherwork piece using conventional paint spraying techniques and can beremoved with standard techniques currently used for pure BN. In apotentially preferred practice, the metal work piece is aluminum ormagnesium although other work piece materials may likewise be used. Theinventive boron nitride-graphite formulations are believed to beparticularly suitable for formation practices carried out attemperatures of about 250 degrees C. or greater.

The substantial benefits in lubricity achieved as a result of graphiteaddition are illustrated in FIG. 6. In this chart coefficient offriction (CoF) is measured for different levels of graphite addition toboron nitride. Coefficient of friction was measured using a CameronPlint reciprocating plate on plate test procedure at 450 degrees C. Thetest was duplicated at each level of graphite addition to confirmrelative performance.

As shown, standard boron nitride lubricant with no graphite addition(far left columns) yielded substantially greater coefficient of frictionlevels than samples with graphite addition. The data also show that withthe addition of graphite lubricity tends to be maintained atsubstantially equivalent levels for longer time periods. This isreflected by the fact that there was no substantial increase incoefficient of friction during latter stages of the friction test.Moreover, a visual examination of samples following the friction testsshowed that pure graphite (far right columns) tended to delaminate tosome degree while the BN graphite combinations maintained substantialadherence. The lubricant blend thus provides the benefit of improvedlubricity in combination with long term adhesion under friction.

It is to be understood that while the present invention has beenillustrated and described in relation to potentially preferredembodiments, constructions, and procedures, that such embodiments,constructions, and procedures are illustrative only and that theinvention is in no event limited thereto. Rather, it is contemplatedthat modifications and variations embodying the principals of theinvention will no doubt occur to those of skill in the art. It istherefore contemplated and intended that the present invention shallextend to all such modifications and variations as may incorporate thebroad aspects of the invention within the true spirit and scope thereof.

1. An elevated temperature metal forming process comprising the stepsof: heating a metal work piece to a temperature of at least 250 degreesC.; applying a lubricant formulation to at least one of (a) a metal workpiece surface and (b) a surface of a formation tool; deforming the metalwork piece within the formation tool to substantially conform with thesurface of the formation tool; and removing the metal work piece fromthe formation tool, wherein the lubricant formulation comprises boronnitride in combination with not less than about 5% graphite.
 2. Theinvention according to claim 1 wherein said metal forming process isquick plastic forming.
 3. The invention according to claim 1 whereinsaid metal forming process is superplastic forming.
 4. The inventionaccording to claim 1 wherein said metal forming process is warm formingusing complementary male and female formation tools.
 5. The inventionaccording to claim 1 wherein said metal forming process is an aluminumforming process.
 6. The invention according to claim 1 wherein saidmetal forming process is a magnesium forming process.
 7. An elevatedtemperature metal forming process comprising the steps of: heating ametal work piece to a temperature of at least 250 degrees C.; applying alubricant formulation to at least one of (a) a metal work piece surfaceand (b) a surface of a formation tool; deforming the metal work piecewithin the formation tool to substantially conform with the surface ofthe formation tool; and removing the metal work piece from the formationtool, wherein the lubricant formulation consists essentially of boronnitride in combination with about 10% to about 50% graphite.
 8. Theinvention according to claim 7, wherein said metal forming process isquick plastic forming.
 9. The invention according to claim 7, whereinsaid metal forming process is superplastic forming.
 10. The inventionaccording to claim 7 wherein said metal forming process is warm formingusing complementary male and female formation tools.
 11. The inventionaccording to claim 7 wherein said metal forming process is an aluminumforming process.
 12. The invention according to claim 7 wherein saidmetal forming process is a magnesium forming process.
 13. An elevatedtemperature metal forming process comprising the steps of: heating ametal work piece to a temperature of at least 250 degrees C.; applying alubricant formulation to at least one of (a) a metal work piece surfaceand (b) a surface of a formation tool; deforming the metal work piecewithin the formation tool to substantially conform with the surface ofthe formation tool; and removing the metal work piece from the formationtool, wherein the lubricant formulation consists essentially of boronnitride in combination with about 5% to about 33% graphite.
 14. Theinvention according to claim 13, wherein said metal forming process isquick plastic forming.
 15. The invention according to claim 13, whereinsaid metal forming process is superplastic forming.
 16. The inventionaccording to claim 13 wherein said metal forming process is warm formingusing complementary male and female formation tools.
 17. The inventionaccording to claim 13 wherein said metal forming process is an aluminumforming process.
 18. The invention according to claim 13, wherein saidmetal forming process is a magnesium forming process.